In order to develop a high performance Mg-Y-Zn alloy containing LPSO phase in the condition of lower Y/Zn atomic ratio, the effect of Ti and Zr on microstructures and mechanical properties of Mg 95 Y 2.5 Zn 2.5 alloy was investigated. Although the a-Mg grains can be refined by adding Ti and Zr separately, the best refinement effect can be obtained by the combined addition of them. More importantly, the formation of LPSO phase was promoted by adding them in as-cast Mg 95 Y 2.5 Zn 2.5 alloy. Meanwhile, the growth model of W phase was transformed from divorced eutectic to cooperative eutectic. In this case, the LPSO/W interface became continuous, which degraded the mechanical properties of as-cast alloys. After extrusion, the increased LPSO phase caused by adding Ti and Zr greatly improved the strength of as-extruded alloy.

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The effects of phase content and evolution on mechanical properties of the alloys are investigated by extruding Mg 95 Y 2.5 Zn 2.5 and Mg 93.1 Y 2.5 Zn 2.5 Ti 1.6 Zr 0.3 alloys with different original microstructures. The 18R-LPSO phase plays a decisive role in the strength of as-extruded alloys. The spherical W-MgYZn 2 phase enhances the deformability and ductility of the alloys. The dynamic recrystallization grains can be observed in all as-extruded alloys. However, the 14H-LPSO clusters and dynamic precipitates only form in the as-cast and T41-treated (solid-solution treatment at 530 C for 3 h with water cooling) alloys after extrusion. Based on synergistic effects of these phases mentioned above, the T41-treated Mg 93.1 Y 2.5 Zn 2.5 Ti 1.6 Zr 0.3 alloy exhibits good compressive mechanical properties after extrusion.

M agnesium alloys as the lightest structure materials have attracted extensive concerns because of their low density, high specific strength, and good shielding characteristics. However, the disadvantages of lower strength and poorer formability in magnesium alloys seriously limit their extensive applications in industry.Composite strengthening is an important means to improve the properties of the alloys. So, it is important to explore an effective strengthening phase. Recently, magnesium alloys with long period stacking ordered (LPSO) structure have attracted much attention due to their excellent mechanical properties. Luo et al [1] firstly identified that the 18R-LPSO phase was identical to the X-Mg 12 YZn phase. It was also found that there are other two phases (I phase and W phase) in Mg-Y-Zn alloys under conventional casting according to the different Zn/ Y atomic ratio [2][3] . Because of the composition fluctuation (segregation) and non-equilibrium solidification, LPSO Abstract: The microstructure evolution of Mg 100-2x Y x Zn x (x=2, 2.5, 3, 3.5) alloys was investigated. Results show that the Mg 100-2x Y x Zn x alloys are composed of α-Mg, long period stacking ordered (LPSO) phase and eutectic structure phase (W phase), and the Mg 95 Y 2.5 Zn 2.5 alloy has the best comprehensive mechanical properties. Subsequently, the microstructure evolution of the optimized alloy Mg 95 Y 2.5 Zn 2.5 during solidification and heat treatment processes was analyzed and discussed by means of OM, SEM, TEM, XRD and DTA. After heat treatment, the lamellar phase 14H-LPSO precipitated in α-Mg and W phase transforms into particle phase (Mg Y Zn 2 ). Due to the compound reinforcement effect of the particle phase and LPSO phase (18R+14H), the mechanical properties of the alloy are enhanced. The tensile strength and elongation of the Mg 95 Y 2.5 Zn 2.5 alloy is improved by 9.1% and 31.3% to 215 MPa and 10.5%, respectively, after solid-solution treatment. phase and W phase can coexist when the value of Zn/ Y (at.%) was 1, as discussed in the study of Li et al [4] . A variety of high strength magnesium alloys containing LPSO phase have been developed. However, there was little research on the Mg-Y-Zn alloys containing LPSO phase and W phase in as-cast condition. Therefore, it is very necessary to study the effect of composite reinforcement of LPSO phase and W phase.Meanwhile, mechanical properties are controlled by the content and morphology of α-Mg, LPSO phase and W phase in the Mg-Y-Zn (atomic ratio Zn/Y=1) alloys. In order to develop a compound reinforced magnesium alloy with LPSO phase and W phase, the present research investigated the effect of different Y and Zn contents on microstructure evolution and mechanical properties of the Mg-Y-Zn alloys with Zn/Y (at.%) ratio of 1. Subsequently, the formation of LPSO phase and W phase in the process of solidification and the effect of the microstructure after solid-solution treatment on mechanical properties in the optimized Mg 95 Y 2.5 Zn 2.5 alloy were studied.

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